After a falling out with the party, labor is back with cash and other support, driven largely by Republican attacks on collective bargaining.

Reporting from Washington — Last May, AFL-CIO President Richard Trumka stood a few blocks from the White House and issued a stern warning: Union members could not be counted on as the Democrats' foot soldiers anymore.

"If leaders aren't blocking the wrecking ball and advancing working families' interests, then working people will not support them," he said in a speech at the National Press Club.

Flash forward to today: Labor appears squarely back in the Democrats' corner for the 2012 election — pushed there in large part by Republican attacks on collective bargaining rights for public employees.

Those and other anti-union measures are rallying organized labor to the side of its longtime Democratic allies, and not just in states such as Wisconsin, Ohio and Michigan, where they are battling efforts aimed at curbing union organizing.

The country's biggest unions also have played a central role in helping a network of federal pro-Democratic "super PACs" get off the ground, pouring more than $4 million into those groups in 2011, even as many wealthy liberals kept their checkbooks closed.

And some major labor groups have even inserted themselves into the Republican presidential primaries with ads that take aim at White House hopeful Mitt Romney.

The decision by unions to act again as an early firewall for Democrats speaks to how stepped-up hostility by Republicans has curtailed labor's hope to be an independent political force.

Across the country, state GOP lawmakers — many of whom were swept into office by the tea-party-fueled wave that dominated the 2010 midterm election — are aggressively pushing right-to-work laws that would make it harder for unions to collect dues. And in the presidential campaign, Romney has taken a particularly antagonistic posture against what he calls "big labor."

"I think we'll be more engaged in 2012 than certainly in the last 20 years," said Mike Podhorzer, political director for the AFL-CIO, a federation of 57 unions. "Working people realize in a way they never have what a threat the current Republican platform is to their well-being."

Organized labor is now expected to match or slightly exceed the estimated $400 million that unions spent to help elect Barack Obama and congressional Democrats in 2008, according to Marick F. Masters, a business professor who studies the labor movement at Michigan's Wayne State University.

The political power of unions was amplified by the Supreme Court's 2010 Citizens United case freeing corporations and unions to do independent political spending. That means that organized labor no longer has to limit its outreach to union members during its vaunted door-to-door field programs. Unions can also pay for political ads directly with dues, rather than through separate political action committees.

So far, however, labor groups have been cautious about exploiting that tool, wary that using member dues for television spots would trigger criticism. For now, union leaders say they plan to focus their political spending on expanding their ground efforts to turn out voters.

One of the biggest unions, the American Federation of State, County and Municipal Employees, plans to put in as much as $100 million this year — money that will largely benefit Obama and other Democrats at the local, state and national levels. "What's the alternative?" Gerald McEntee, the federation's president, told the Washington Post in October.

Last month, AFSCME spent $1 million in Florida on a television ad attacking Romney for his ties to a medical company that admitted to defrauding Medicare — the first time the union had weighed in during a Republican primary. The Service Employees International Union, working with the pro-Obama super PAC Priorities USA Action, ran its own anti-Romney radio ads in Florida and Nevada.

Labor officials insist their recent political activity does not mean they are a de facto arm of the Democratic Party, a frequent charge made by conservatives. Larry Scanlon, AFSCME's political director, noted that the union backed GOP officials at the local and state level.

But the political climate has made it increasingly difficult to find pro-labor Republicans on a national level.

"We're trying, because we want to work with anybody who wants to work with us, but the options have not been great," said Brandon Davis, political director for the service employees' union.

The aggression on the right led to a shift in labor's strategy from what it was less than a year ago, when union leaders complained that Democrats were not fighting hard enough for their issues. They ticked off a litany of complaints, including the failure of Congress to pass "card check" legislation that would make union organizing easier and Obama's support for a trade deal with Colombia, where union members have been killed.